EU leaders last December approved the broad outline of Juncker's €315 billion investment plan, intended to kick-start growth in Europe by financing hundreds of projects.
Renzi said on Twitter that Italy was pledging "eight billion euros" to the programme, matching a commitment from France this month and €1.5 billion promised by Spain at the end of February.
Germany has also committed €15 billion to the plan, though other EU member states have been slower to react.
"The ball is rolling! Italy is the fourth member state to pledge a sizeable contribution to our Investment Plan for Europe," European Commission vice-president Jyrki Katainen said.
"I am delighted that member states are putting their money where their mouths are, helping us to make the most of the plan and to create jobs and lasting growth in Europe," said Katainen, who is responsible for growth and investment.